Separation membranes are used in many different fields, such as the water treatment field including seawater desalination, water purification and drainage treatment; the medical field including blood purification; the food industry field; and the electrical industry field including battery separators, charged membranes and fuel cells. Such separation membranes generally comprise a woven or nonwoven fabric base (sheet) and a separation layer having a separation function, which is formed directly over the base. For example, JP 2007-283287 A discloses a separation membrane comprising a polyvinylidene fluoride-based separation layer formed over a nonwoven polyester base, and JP 2003-245530 A and JP 09-313905 A disclose separation membranes comprising a polysulfone separation layer formed over a nonwoven polyester base.
Such separation membranes are produced by applying a polymer solution for separation layer formation to a surface of an elongated base and then passing the base through a solidifying step and a cleaning step to form a separation layer directly over the surface of the base. In doing so, the base and the separation layer have different chemical compositions and different thermal shrinkage ratios from each other, and this causes a phenomenon to occur in which both widthwise ends of the base curl.
This curling phenomenon is explained in FIG. 12a, FIG. 12b and FIG. 12c. FIG. 12a is a cross-sectional view of an elongated base 121 immediately after a polymer solution coating film 122 has been formed over the surface of the base 121 subsequent to a polymer solution for separation layer formation being applied to the base 121. In FIG. 12a, the crosswise direction of the base 121 is the width direction thereof, and the longitudinal direction of the base 121 is perpendicular to the page. The base 121 coated with the polymer solution is subsequently soaked in a solidifying solution to form a solidified polymer solution coating film 122. Next, the base 121 having the polymer solution coating film 122, which has been solidified thereon, is washed with water and cleaned, and a separation membrane comprising a base and a separation layer is finally produced.
In the solidifying step and the water-washing step, shrinkage occurs in both the base 121 and the polymer solution coating film 122. In respect of the occurrence of shrinkage, the shrinkage of the polymer solution coating film 122 is greater than that of the base 121, and accordingly, the shrinkage difference between the base 121 and the polymer solution coating film 122 becomes obvious at both widthwise end portions of the base 121, causing the base 121 to curl.
FIG. 12b is a cross-sectional view of the base 121 in the initial state with curls 123 beginning to develop at both end portions of the base 121. FIG. 12c is a cross-sectional view of curls 123 that are further rolled 124 as both ends of the base 121 are brought inwards.
The occurrence of the curls 124 shown in FIG. 12c in both end portions of the base 121 significantly reduces the operability of the production and element formation processes of separation membranes.